Microstructure evolution and strengthening mechanisms of pure titanium with nanostructured

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Accepted Manuscript

Microstructure evolution and strengthening mechanisms of pure titanium with nano-

structured surface obtained by high energy shot peening

Shijuan Dai, Yuntian Zhu, Zhaowen Huang

PII: S0042-207X(16)30001-X

DOI: 10.1016/j.vacuum.2016.01.001

Reference: VAC 6899

To appear in: Vacuum

Received Date: 27 November 2015

Revised Date: 31 December 2015

Accepted Date: 3 January 2016

Please cite this article as: Dai S, Zhu Y, Huang Z, Microstructure evolution and strengthening

mechanisms of pure titanium with nano-structured surface obtained by high energy shot peening,

Vaccum (2016), doi: 10.1016/j.vacuum.2016.01.001.

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http://dx.doi.org/10.1016/j.vacuum.2016.01.001



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Microstructure evolution and strengthening mechanisms of pure

titanium with nano-structured surface obtained by high energy shot

peening

Shijuan Daia,

*, Yuntian Zhub, Zhaowen Huang

a

a Nano Structural Materials Center, Nanjing University of Science and Technology,

Nanjing 210094, China

b College of Engineering, North Carolina State University,

North Carolina State 27616, USA

Abstract: In this study, the microstructure evolution and strengthening mechanisms

of pure titanium processed by high energy shot peening (HESP) have been studied.

The results show that the deformation layer is formed on the surface and the

microstructure exhibits with the equiaxed 20~40µm grains in the matrix after HESP. A

nanocrystal surface layer is produced by means of HESP on pure titanium. The

formation of nano-grains on the surface can be separated into four steps: (1) the

formation of the dislocations tangles; (2) the occurrence of the intersection of twins;

(3) appearance of slip band and subgrains; (4) formation of uniformly distributed

nanometer-scale grains. With increasing the holding time, the strength increases and

the elongation decreases due to the work hardening effect and the formation of the

nanocrystals on the surface.

Key words: Pure titanium; Gradient nanocrystallization; High energy shot peening;

Microstructure; Tensile properties

1.1Introduction

Titanium and its alloys have become excellent metal biomaterials due to their

*Corresponding author. Tel.: +008613814088460.

E-mail address: [email protected] (Shijuan Dai).

Address: Nano Structural Materials Center, Nanjing University of Science and Technology, 200 Xiao Ling Wei, Nanjing 210094, China.



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lower modulus, superior biocompatibility and enhanced corrosion resistance

compared with stainless steels and cobalt-based alloys [1, 2]. So far, the most widely

used alloy is pure titanium and Ti-6Al-4V alloy for clinical application, but the latter

contains V and Al elements, which exhibit high cytotoxicity [3]. Pure titanium

without toxic elements is considered as the one of the best implanted materials [4-5].

However, the strength of pure titanium is lower than that of Ti-6Al-4V, leading the

narrower application scope.

The surface nanocrystallization (SNC) is an effective and economical route to

improve the mechanical properties of metal materials. A variety of severe plastic

deformation processes have been proposed to produce SNC. Shot peening treatment,

which can produce nano-grains in the surface layer of the metals, is an effective

surface strengthening technology. During the process, the surface of the workpiece is

continuously impacted by a number of shot, which causes the severe plastic

deformation. Unal et al. [6] studied the microstructure and hardness of AISI 304

austenitic stainless steel performed by different types of shot peening, including air

blast conventional shot peening, severe shot peening and repeening. The results

showed that the deformation layer had nano-grain size distributions with much higher

hardness. Bagherifard et al. [7] studied the microstructure and roughness of cast iron

specimens treated by severe shot peening and the results indicated the presence of a

highly deformed near surface layer and the surface roughness increases with

increasing the impact energy of shot peening process. The above literatures

have focused on the microstructure evolution of metal materials after shot peening,

but few investigations about the influences of the microstructure evolution on

mechanical properties after shot peening have been founded.

In this study, a nano-structured grade surface layer was prepared on the surface of

pure titanium by means of HESP technique in order to enhance the strength. The

mechanism of grain refinement was discussed and the effect of the formation of the

nano-grains on the mechanical properties was also studied.

2. Experimental

The material used in this study was a 4mm-thickness plate made of pure titanium



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with the chemical composition in weight percent: 0.10Fe, 0.14O, 0.25Al and balance

Ti. The plate was annealed in argon atmosphere at 1023K for 2h and then air-cooled,

resulting in equiaxed grains averaging 30µm in size. The shot velocity was 40m/s and

the stainless steel balls with the 3mm diameter were used as the shots g. Both sides of

the annealed samples were high energy shot peened at room temperature and the

holding time was 15, 30, 45 and 60min respectively.

The cross-section of each sample was mechanically polished using silicon

carbide paper and a polishing cloth, and finally etched at room temperature in a

solution of 1 ml HF, 3 ml HNO3 and 7 ml distilled water. Then, the microstructure

close to the treated surface was examined by optical microscope (OM). The phase

structure of the surface layer was performed using an X-ray diffractometer (XRD, D8

Discover) with Cu Kα radiation and graphite monochromator operated at 40KV and

40mA. The micro-strains and average grain sizes of all the samples were calculated

according to the XRD pattern using Jade5.0 software. Transmission electron

microscopy (TEM) investigations were carried out on FEI Tecnai 20 microscopes

operating at 200 KV. The preparation of the TEM foils of the cross-sections was

separated into four steps: (1) polishing the cross-section of the sample mechanically

until it was about 50µm thick (Fig. 1(a)); (2) cutting along the dotted line (Fig. 1(b));

(4) sticking on a copper ring with the butt joint of the two treated surface (Fig. 1(c)

and (d)); (4) electro-polishing the foils using a twin-jet technique in a solution of 6ml

HClO4, 60ml CH3OH and 36ml C4H9OH at a voltage of 30V and a temperature of

243K. The mechanical properties of the alloys were obtained by an electronic

universal test machine (CMT 5105). The size of the sample was shown in Fig.2 and

the tensile specimens were prepared by using electro-discharging.

3. Results and discussions

3.1 Microstructures

The cross-sectional microstructures of the treated specimens under different

conditions are presented together in Fig. 3. It is seen that the grain boundaries have

not been indentified and it is called the severe deformation layer which is marked in



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dotted line. Also, it is observed that the interior is not affected by the deformation and

the microstructure exhibits with the equiaxed 20~40µm grains. From Fig. 3, it is

known that the thickness of the severe deformation layer increases obviously with the

increase of the holding time. When the holding time increases, the accumulation of

severe deformation causes by the repeated impact of the shots enhanced, leading to

the increase of the thickness of the severe deformation layer [8].

3.2 XRD investigations

Fig. 4 shows XRD patterns of the samples treated for different holding time.

No new peaks appear after HESP, which shows that there is no obvious phase

transition after severe plastic deformation. Compared with the specimen before

HESP, the diffraction peaks broaden after HESP and the widening of diffraction line

profile intensifies with the increase of the holding time, which infers the occurrence of

grain refinement and the increasing micro-strain.

The grain size and micro-strain of the samples are calculated from line

broadening of Bragg diffraction peaks by using equation (1) and (2) respectively [9

10].

(1)

(2)

Where Size means grain size; K means constant, usually K=1; λ means X ray

wavelength; β means the full width at half maximum of the peak; θ is the Bragg angle

of the [h k l] reflection; ε means micro-strain. Fig. 5 shows the average grain size and

the micro-strain before and after HESP and it can be seen that the average grain size

of the sample decreases and the micro-strain strengthens with the increase of the

holding time. Average grain size reduction below 100nm can be evaluated after

treating for 60min and the mechanism of the grain refinement will be studied in next

part. After HESP, a lot of defects are introduced into the surface, which causes the

enhancement of the lattice distortion and increased micro-strain.

Size= cos

tan4

K λ θ

β

β ε θ =



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3.3 TEM investigations

Fig. 6 shows the TEM images of the sample treated for 15min. The typical

microstructure feature is dislocation tangles, as shown in Fig. 6(a). The irregular

dislocation cells with different sizes (100~500 nm) appear and they are tangled with

each other in a complex way. As shown in Fig. 6(b), the corresponding selected area

electron diffraction (SAED) shows no disorientations inside the tangles. Meanwhile,

the mechanical twins are not observed, which shows that only the formation of the

dislocations occurs at the beginning of the deformation process.

When the holding time increases to 30min, a larger number of parallel twins can

be observed (Fig. 7(a)) and the SAED patterns are given in Fig. 7(b). It is concluded

that they are the twins with respect to (0 5 -5 12) plane by analyzing Fig. 7(c). It was

reported that the twins on {1 0 -1 2}, {1 1 -2 1}, and {1 1 -2 2} planes had formed

during deformation at room temperatures for α-Ti [11-14]. For example, Zhu et al [15]

founded that the twins with respect to (1 0 -1 2) plane appeared after surface

mechanical attrition treatment. However, the mechanical twins with respect to (0 5 -5

12) plane have not founded in other literatures and the detailed mechanisms have not

been clear until now. Also, the intersection of strip twins is observed beside the

parallel twins (Fig. 7(e)). One twin might end at the other one (such as T1 and T3) or

go across the other one (such as T3 and T4), as a result of a shear accommodation

process. Maybe it is because a large number of dislocations generates at the

beginning of the deformation and then the dislocation density increases dramatically.

When the dislocation density increases to a certain extent, the deformation process is

difficult to continue. At this time, the formation of the mechanical twins, which can

often change the hard orientation into soft orientation, are believed to accommodate

the increasing strain during deformation so that the subsequent sliding deformation

occurs more easily [16-18].

Fig. 8 shows TEM images of the sample treated for 45min.

Thereby causing

greater strain, a larger number of parallel slip bands can be observed (Fig. 8(a)). Their

boundaries are not as straight as those of the twins (the site of white arrows in Fig.



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8(a)) and a lot of dislocation pile-ups are present in the bands. Meanwhile, it can be

seen that the typical microstructure in the slip band are subgrains with the size of

about 200nm (the site of white arrows in Fig. 8(b)). It is proved that high angle

misorientations among the subgrains appear and the misorientation between the

subgrains and surrounding is about 22°, suggesting the transition from low to high

angle gain boundary at this step (seen Fig. 8(c)).

When the holding time increases to 60min, the microstructure is characterized by

uniformly distributed nanometer-scale grains (seen Fig. 9(a)) and the corresponding

SAED patterns (Fig. 9(b)) exhibits the formation of finer grains with a more random

orientation on the top treated surface. With the increase of the strain, the defect

density and the stored energy increase, leading to the lower recrystallization

temperature and dynamic recrystallization. Then the nano-grains with randomly

orientation on the surface layer are formed [19, 20].

3.4 Mechanical properties

Fig. 10 shows the mechanical properties of the samples before and after

HESP. The ultimate

tensile strength (UTS) and the yield strength of the sample before

HESP and after HESP for 60min are 545MPa and 637MPa, 405MPa and 540MPa,

and they increases by 17% and 33%, respectively (seen Fig. 10(a)). The increase in

strength after HESP can be attributed to two primary factors. On one hand, the

introduction of the defects on the surface leads to the increase of the strength. During

shot peening process, the severe deformation occurs on the surface of the sample and

a large number of dislocations and twins are formed, resulting in the work hardening

[21, 22]. The thickness of the deformation layer increases with prolonging the holding

time, leading to the increase of the strength. On the other hand, the grain refinement

of the surface also contributes to the variety of the mechanical properties with the

increase of the holding time. The average grain size of the sample before HESP is

30µm and it is gradually refined to nano-scale after HESP for 60min.

In addition, it is seen that the elongation before and after HESP for 60min was 31%

and 18% and it decreased by 40% after treatment from Fig. 10(a). However, the



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elongation of the sample treated for 60min is still far larger than 8%, which meets

the requirements of biomedical metal materials [23-25]. Work hardening is a factor

for the decrease of the elongation with the increase of the holding time. The

introduction of a large number of defects results in the difficulty of the deformation.

The information of nano-grains is another factor for the worse of the elongation.

Some researchers reported that the deformation occurred by grain boundary sliding

and grain rotation instead of dislocation sliding for nano-grains [26-28].There is no

space for the dislocation gliding of nano-grains due to the small grain size and the

macroscopic measurable deformation cannot be accumulated, resulting the wore

ductility.

Fig. 10(b) depicts the stress-strain curves of the samples and it can see that the

climbing rate at the hardening stage of the curves enhances with the increase of the

holding time, which means that the work hardening effect strengthened. In addition,

the local plastic deformation stages are observed in the curves for all the samples. It

shows that the facture has gone through a process and it cannot rupture immediately

when it reaches the peak of the strength, which infers that the samples have

undergone the process of necking and the fracture behavior belongs to ductile

fracture.

4. Conclusions

1. After HESP, the deformation layer was formed on the surface and the

microstructure exhibited with the equiaxed 20~40µm grains in the matrix. With

prolonging the holding time, the thickness of the deformation layer increased.

2.

A nanocrystal surface layer was produced on pure titanium by HESP. The

microstructures evolution could be separated into four steps: (1) the formation of the

dislocations tangles; (2) the occurrence of the intersection of twins; (3) appearance of

the slip band and subgrains; (4) formation of uniformly distributed nanometer-scale

grains.

3. With the increase of the holding time, the strength increased and the elongation

decreased due to the work hardening effect and the formation of the nano-grains on

the surface.



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Acknowledgement

This work was partly supported by Jiangsu Planned Projects for Postdoctoral

Research Funds (grant of 1402008A).

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Figure Captions

Fig. 1 Preparation process of TEM foils

Fig.2 Size of tensile test specimen

Fig.3 Cross-sectional optical micrographs close to the treated surfaces for different

holding time: (a) 15min; (b) 30min; (c) 45min; (d) 60min

Fig.4 XRD patterns of the samples before and after HESP

Fig. 5 Average grain sizes and micro-strains of the samples before and after HESP

Fig.6 TEM images of the sample treated for 15min (a) bright field image; (b) SAED

pattern of (a)

Fig.7 TEM images of the sample treated for 30min (a) parallel twins; (b) SAED

pattern of (a); (c) sketch map of (b); (d) intersection of the twins; (e) SAED pattern of

(d)

Fig.8 TEM images of the sample treated for 45min: (a) slip bands; (b) subgrains;(c)

SAED pattern of (b)



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Fig.9 TEM images of the sample treated for 60min: (a)bright field image; (b)SAED

pattern of (a)

Fig. 10 (a) Mechanical properties and (b) stress-strain curves of the samples before

and after HESP



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Fig. 1

Fig. 1 Preparation process of TEM foils



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Fig.2

Fig.2 Size of tensile test specimen



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Fig. 3

Fig.3 Cross-sectional optical micrographs close to the treated surfaces for different holding

time: (a) 15min; (b) 30min; (c) 45min; (d) 60min



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Fig. 4

0100020003000

0

100020003000

0100020003000

0 20 40 60 80 100

010002000

3000

0100020003000

I n t e n s i t y

( 1 1 0 )

( 1 0 3 )

( 1 1 2 )Before HESP

( 1 0 0 )

( 0 0 2 )

Treated for 60min

Treated for 45min

2theta/deg

( 1 0 1 )

( 1 0 2 )

Treated for 30min

Treated for 15min

Fig.4 XRD patterns of the samples before and after HESP



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Fig. 5

0 10 20 30 40 50 60

0

5000

10000

15000

20000

0.0

0.1

0.2

0.3

0.4

Mi c r o- s t r a i n / %

A v e r a g e g r a i n s i z e / n m

Time/min

Grain size

Micro-strain

Fig. 5 Average grain sizes and micro-strains of the samples before and after HESP



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Fig. 6

Fig.6 TEM images of the sample treated for 15min (a) bright field image; (b) SAED pattern

of (a)



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Fig. 7

Fig.7 TEM images of the sample treated for 30min (a) parallel twins; (b) SAED pattern of

(a); (c) sketch map of (b); (d) intersection of the twins; (e) SAED pattern of (d)



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Fig. 8

Fig.8 TEM images of the sample treated for 45min: (a) slip bands; (b) subgrains;(c) SAED

pattern of (b)



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Fig. 9

Fig. 9 TEM images of the sample treated for 60min: (a)bright field image; (b)SAED pattern

of (a)



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Fig. 10

Fig. 10 (a) Mechanical properties and (b) stress-strain curves of the samples before and

after HESP



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A nanocrystalline surface layer was produced by HESP on pure titanium.

The formation of nano-grains on the surface can be separated into four steps.

With increasing the holding time, the strength increased due to work hardening.

Microstructure evolution and strengthening mechanisms of pure titanium with nanostructured

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